Bipolar electrodes with high energy efficiency, and use thereof for synthesising sodium chlorate

ABSTRACT

The invention relates to novel bipolar electrodes with a cathodic coating on one portion of the electrode and an anodic coating on another portion of the same electrode. The anodic coating is preferably a DSA coating and the cathodic coating is an alloy such as Fe 3−x Al- 1+x M y T z . The invention also relates to the use of said novel electrodes for synthesising sodium chlorate.

FIELD OF THE INVENTION

The present invention relates to novel bipolar electrodes with a cathodic coating on a part of it and an anodic coating on another part of it. It also relates to the usage of these novel electrodes for the synthesis of sodium chlorate.

BRIEF DESCRIPTION OF THE DRAWINGS

In the attached drawings:

FIGS. 1 a and 1 b are schematic views of mono-polar electrodes;

FIGS. 2 a and 2 b are schematic views of bipolar electrodes;

FIG. 3 is an illustration of an adhesion test of a coating of iron aluminide on steel 1020;

FIG. 4 is an illustration of an adhesion test of a coating of iron aluminide on titanium;

FIG. 5 is a scheme illustrating the assembly of traction dowels used for the adhesion tests;

FIG. 6 is an illustration of a corrosion test in a chlorate solution of a DSA electrode and of a coating of the type Fe_(3−x)Al_(1+x)M_(y)T_(z) on a substrate of titanium;

FIG. 7 is a schematic view of a bipolar electrode in accordance with the invention;

FIGS. 8 a, 8 b are schematic views of bipolar modules in accordance with the invention; and

FIGS. 9 a, 9 b and 9 c are photographs of bipolar electrodes fabricated in such a manner that a part of these electrodes is covered by a coating of the DSA type and another part by a coating of the type Fe_(3−x)Al_(1+x)M_(y)T_(z).

TECHNOLOGICAL BACKGROUND

Sodium chlorate (NaClO₃) is currently used as bleaching agent in the pulp and paper industry. It is produced by electrolysis of sodium salt (NaCl) in accordance with the chemical reaction:

NaCl+3H₂O→NaClO₃+3H₂

The process is very energy-consuming and requires between 5000 and 5500 kW of electricity per ton of sodium chlorate. The electrolysis cells in which a high continuous current circulates customarily comprises anodes that are dimensionally stable (DSA) and uncoated cathodes of steel or of titanium. DSA anodes are well-known in the art of electrolysis cells, see, for example: WO4101852, WO4094698, U.S. Pat. No. 6,071,570, U.S. Pat. No. 4,528,084, U.S. Pat. No. 5,989,396, U.S. Pat. No. 6,572,758, U.S. Pat. No. 4,233,340; U.S. Pat. No. 5,419,824; U.S. Pat. No. 5,593,556 and U.S. Pat. No. 5,672,394.

These DSA anodes typically comprise a substrate of titanium on which a coating of ruthenium oxide is applied possibly with other oxides or compounds such as iridium oxide. By virtue of this catalytic coating the energy losses on the anodic side are low. This is reflected by a low anodic overvoltage several tens of millivolts. However, it is not the same on the cathodic side. The cathodic overvoltage on the surface of a steel plate is approximately 900 mV whereas on the surface of a plate of titanium it is approximately 1200 mV. The energy losses on the cathodic side thus represent the main source of energy losses in the process. It is for this reason that in the course of recent years the inventors of the present invention attempted to find performing cathode coatings that allow the overpotential on these electrodes to be lowered.

WO/2008/138148 which also originates from the inventors of the present invention, gives an example of such cathode coatings. It describes alloys of the type Fe_(3−x)Al_(1+x)M_(y)T_(z) that are applied on the surface of an electrode for making a coated cathode that is very performing in regards to energy.

Cathodes and anodes are assembled in electrolysis cells according to different configurations. Two types of assembly are distinguished. The mono-polar cells and the bipolar cells. FIG. 1 shows schematic views of mono-polar electrodes. In such configurations each electrode only plays one role, that of anode or of cathode. Consequently, there is no ambiguity about the type of coating to be applied if one wishes to improve the energy efficiency of such cells. For the anode, a substrate of titanium will be selected and a coating of ruthenium oxide will be applied in order to make a DSA and for the cathode a steel plate could be selected and a coating of the type Fe_(3−x)Al_(1+x)M_(y)T_(z) could be applied in order to make a cathode with high energy performance.

FIG. 2 shows schematic views of bipolar electrodes. In a bipolar configuration an electrode or a module of electrodes simultaneously plays the role of anode and that of cathode. In the scheme at the top of FIG. 2 a the negative face of the bipolar electrode is cathodic while the positive face is anodic. In the scheme at the bottom of FIG. 2 b the electrodes in the left part of the bipolar module (negative sign) are cathodic while the electrodes on the right side (positive sign) are anodic. These electrodes are assembled and welded together in order to make a bipolar module of electrodes. Since a bipolar electrode such as the one shown in FIG. 2 a simultaneously plays the role of anode and cathode, which type of electrode should be selected in order to globally improve the efficiency of the process? Should one choose a DSA electrode on a substrate of titanium that was developed for optimizing the anodic reaction or a steel plate with catalytic coating in order to favor the cathodic reaction? In addition to this difficulty, a bipolar module of electrodes such as the one shown at the bottom of FIG. 2 presents an additional problem. The electrodes on the anodic side (right side of the module) are customarily DSAs on substrates of titanium whereas the electrodes on the cathodic side (left side of the module) are steel plates. Now, it is very difficult to weld titanium to steel. Such a module therefore presents a difficulty in the assembly.

Finally, when different metals such as steel and titanium are in direct contact in a highly corrosive solution such as that of sodium chlorate, there is an additional problem of galvanic corrosion. When production stops and the current is cutted in a plant, a current caused by the galvanic corrosion circulates in the opposite direction in the modules of bipolar electrodes and this effect causes a severe deterioration of the less noble electrodes.

The present invention has the goal of solving these problems associated with bipolar electrodes.

SUMMARY OF THE INVENTION

When they did their research about the cathodic coatings with high energy performance of the type Fe_(3−x)Al_(1+x)M_(y)T_(z) that constitute the subject matter of the invention WO/2008/138148, the inventors of the present invention found to their great surprise that the coatings of this type adhere as well to substrates of steel as to substrates of titanium.

The invention therefore has, as first subject matter, a bipolar electrode with high energy efficiency, which electrode has a part provided with a cathodic coating and another part that is distinct from the first one and that is provided with an anodic coating.

In the invention as claimed:

the anodic coating is of the DSA type; and

the cathodic coating consists of an alloy with the formula:

Fe_(3−x)Al_(1+x)M_(y)T_(z)

in which:

M represents one or several catalytic species selected from Ru, Ir, Pd, Pt, Rh, Os, Re, Ag and Ni;

T represents one or several elements from Mo, Co, Cr, V, Cu, Zn, Nb, W, Zr, Y;

Mn, Cd, Si, B, C, O, N, P, F, S, Cl and Na;

x is a number greater than −1 and lower than or equal to +1;

y is a number greater than 0 and lower than or equal to +1; and

z is a number between 0 and +1.

The substrate on which the coatings are applied can be a substrate of steel or a substrate of titanium.

The invention also has as subject matter a bipolar module of electrodes containing several electrodes such as those described above.

The invention also has as subject matter the use of the bipolar electrode or of the bipolar module in accordance with the invention for the electrosynthesis of sodium chlorate.

EXAMPLES

FIG. 3 shows an adhesion test of the coating of the type Fe₃Al on a substrate of steel 1020 according to ASTM C633. The fracture took place at a stress of 11,922 psi, that is quite close to the fracture limit of the glue serving for the mounting of the dowels (see the scheme of FIG. 5). Thus, the adhesion of a coating of iron aluminide on a steel substrate is excellent.

FIG. 4 shows a similar test for the adhesion of the coating of the same type on a substrate of titanium. The fracture took place at a stress of 10,604 psi, that is, a value almost as high as the previously measured one. Consequently, the adhesion of the coating is just as good on a substrate of titanium as on a substrate of steel.

Since titanium customarily serves as substrate for coatings of the DSA type, this discovery opens the possibility of applying a DSA coating on one side of the substrate of titanium for the anodic reaction and on the other side a coating of the Fe_(3−x)Al_(1+x)M_(y)T_(z) type for the cathodic reaction. In other words, this discovery leads directly to the energy optimization of electrodes of the bipolar type.

However, it is possible to also use a substrate of steel, preferably a stainless steel of the ferritic type not containing Ni. In this case a layer of Ti is preferably applied on one side by a method such as “cold spray” before applying the DSA coating on the same side and on the layer of Ti. A coating of the type Fe_(3−x)Al_(1+x)M_(y)T_(z) is applied on the other side as previously but this time on steel.

The only potential problem remaining in such electrode configurations is that of galvanic corrosion caused by the fact that there is on one side of the electrode an oxide of ruthenium of the DSA type and on the other side an alloy of the type Fe_(3−x)Al_(1+x)M_(y)T_(z). Now, it was discovered that it was possible to adjust the chemical composition of alloys of the type Fe_(3−x)Al_(1+x)M_(y)T_(z) by a judicious choice of the elements M and T and of the compositions x, y and z in such a manner as to balance out the potentials with respect to the DSA and to cancel the galvanic corrosion of the couple constituting the bipolar electrode.

FIG. 6 shows the “current-voltage” curves in a chlorate solution at 22° C. measured relative to a reference electrode Ag/AgCl by sweeping the potential 5 mV/sec for a DSA electrode and a coating of the type Fe_(3−x)Al_(1+x)M_(y)T_(z) on a substrate of titanium. We observe that the cathodic coating is just as resistant to corrosion as the DSA. The corrosion threshold is approximately 1.2 V. The galvanic couple between these dissimilar materials is thus reduced by an appropriate selection of the chemical composition of the coating based on iron aluminide.

Without being restrictive, FIG. 7 shows schematic views of bipolar electrodes in accordance with the invention. For the first electrode, one face has an anodic coating while the other face has a cathodic coating. In the second bipolar electrode, one end of the electrode is covered on two sides by a cathodic coating whereas the other end is covered by an anodic coating.

Without being restrictive, FIG. 8 shows schematic views of bipolar modules constituted by an assembly of bipolar electrodes represented in FIG. 7.

FIGS. 9 a and 9 b show photographs of bipolar electrodes such as those represented schematically in FIG. 7, and FIG. 9 c shows the appearance of a bipolar electrode in accordance with the invention after an immersion of 69 hours in a chlorate solution at 22° C. A beginning of pitting corrosion is observed on the cathodic part but the structural integrity of the coating is still excellent. 

1. A bipolar electrode with high energy efficiency, which electrode has a part provided with a cathodic coating and a second part that is distinct from the first one and is provided with an anodic coating.
 2. A bipolar electrode according to claim 1, wherein the anodic coating is of the DSA type.
 3. A bipolar electrode according to claim 1, wherein the anodic coating is an alloy with the formula: Fe_(3−x)Al_(1+x)M_(y)T_(z) in which: M represents one or several catalytic species selected from Ru, Ir, Pd, Pt, Rh, Os, Re, Ag and Ni; T represents one or several elements from Mo, Co, Cr, V, Cu, Zn, Nb, W, Zr, Y; Mn, Cb, Si, B, C, O, N, P, F, S, Cl x is a number greater than −1 and lower than or equal to +1; y is a number greater than 0 and lower than or equal to +1; and z is a number between 0 and +1.
 4. A bipolar electrode according to claim 1, wherein the coatings are applied on a substrate of steel or of titanium.
 5. A bipolar module of electrodes, comprising an electrode unit containing a plurality of electrodes in accordance with claim
 1. 6. (canceled)
 7. (canceled)
 8. The method of synthesizing sodium chlorate which comprises electrolysis with the bipolar electrode according to claim
 1. 9. The method of synthesizing sodium chlorate which comprises electrolysis with the bipolar module of claim
 5. 10. A bipolar electrode according to claim 3, wherein the coatings are applied on a substrate of steel or of titanium.
 11. A bipolar module of electrodes, comprising an electrode unit containing a plurality of electrodes in accordance with claim
 3. 12. A bipolar module of electrodes, comprising an electrode unit containing a plurality of electrodes in accordance with claim
 4. 13. A bipolar module of electrodes, comprising an electrode unit containing a plurality of electrodes in accordance with claim
 10. 